8S-IgM is found in the sera of patients with such disorders as rheumatoid vasculitis and systemic lupus erythematosus. We have recently developed a sensitive radioimmunoassay that permits the demonstration of minute amounts of this protein in normal sera as well. We plan to investigate the origin of the protein in health and disease, in particular to determine whether it is shed from the surface of lymphocytes, or secreted, as well as to provide information about the possible identity of the 8S-IgM in serum and the receptor-immunoglobulin on lymphocytes. The cytophilic properties of 8S-IgM for Fc receptors on monocytes and lymphocytes will be defined. These studies will be used to test the hypothesis that 8S-IgM serves to mediate an amplification mechanism for the generation of specific antibody, by binding to macrophages and thus facilitating the presentation of antigen to B-cells. The stimulation of B-cells may result in the increased release of 8S-IgM, and in the production of specific antibody; in certain human disorders these events may be associated with exacerbations of disease activity.